Many patients have visual errors associated with the refractive properties of the eye such as nearsightedness (myopia), farsightedness (hyperopia) and astigmatism. Nearsightedness can occur when light focuses before the retina, and farsightedness can occur with light refracted to a focus behind the retina. Astigmatism may occur when the corneal curvature is unequal in two or more directions. These visual impairments are commonly corrected using eyeglasses or contact lenses.
Alternatively, the cornea of the eye can be reshaped surgically to provide the needed optical correction. Eye surgery has become commonplace with some patients pursuing it as an elective procedure to avoid using contact lenses or glasses to correct refractive problems, and others pursuing it to correct adverse conditions such as cataracts. With recent developments in laser technology, laser surgery is becoming the technique of choice for ophthalmic procedures. The reason many eye surgeons prefer a surgical laser beam over manual tools like microkeratomes and forceps is that the laser beam can be focused precisely on extremely small amounts of ocular tissue, thereby enhancing accuracy and reliability of the procedure. These in turn enable better wound healing and recovery following surgery.
Different laser eye surgical systems use different types of laser beams for the various procedures and indications. These include, for instance, ultraviolet lasers, infrared lasers, and near-infrared, ultra-short pulsed lasers. Ultra-short pulsed lasers emit radiation with pulse durations as short as 10 femtoseconds and as long as 3 nanoseconds, and a wavelength between 300 nm and 3000 nm. Examples of laser systems that provide ultra-short pulsed laser beams include the Abbott Medical Optics iFS Advanced Femtosecond Laser System, the IntraLase FS Laser System, and OptiMedica's Catalys Precision Laser System.
Prior surgical approaches for reshaping the cornea include laser assisted in sine keratomileusis (hereinafter “LASIK”), photorefractive keratectomy (hereinafter “PRK”) and Small Incision Lens Extraction (hereinafter “SmILE,”).
LASIK and PRK are currently the two most commonly performed myopia correction procedures. In the LASIK procedure, an ultra-short pulsed laser is used to cut a conical flap that is folded open to expose the corneal stroma for photoablation with ultraviolet beams from an excimer laser. Photoablation of the cortical stroma reshapes the cornea and corrects the refractive condition such as myopia, hyperopia, astigmatism, and the like. After the corneal stroma has been photoablated with the excimer laser, the corneal flap is folded back and closed.
LASIK has the advantages of providing nearly immediate improvement in vision with a minimal amount of pain. There are, however, some disadvantages. Because a void is often created under the flap, the reshaped cornea is structurally weaker after surgery. To address this, LASIK to candidates are typically limited to those patients who have corneas that are about 500-600 μm thick. Further, LASIK requires an ultra-short pulsed laser to cut the cortical flap and a separate excimer laser to ablate the corneal stroma. As would be expected, requiring multiple laser systems increases costs and requires bigger storage areas for the equipment.
It is known that if part of the cornea is removed, the pressure exerted on the cornea by the aqueous humor in the anterior chamber of the eye will act to close the void created in the cornea, resulting in a reshaped cornea. By properly selecting the size, shape and location of a corneal void, one can obtain the desired shape, and hence, the desired optical properties of the cornea.
Hence, recently surgeons have started using another surgical technique for refractive correction. Instead of ablating corneal tissue with an excimer laser following the creation of a corneal flap, the newer technique involves tissue removal with two femtosecond laser incisions that intersect to create a lenticule for extraction. Lenticular extractions can be performed either with or without the creation of a corneal flap. With the flapless procedure, a refractive lenticule is created in the intact portion of the anterior cornea and removed through a small incision. In a SmILE procedure, as shown in FIG. 10, a femtosecond laser is used to make a side cut 1010, upper surface cut 1020 and lower surface cut 1030. A tweezer, for example, is then used to extract the cut lens 1040 beneath the anterior surface of the cornea 1000 through the side cut 1010.
PRK is another alternative to the LASIK procedure. In the PRK procedure, an excimer laser is used to directly remove material from a cornea without creating a flap. First, an epithelium layer of the cornea is removed prior to laser ablation. The epithelium layer will regrow within a few days after the procedure. As shown in FIG. 11, an initial cornea shape 1100 is reshaped by an excimer laser. Based on a desired myopic correction, a small amount of thin-lens shaped cornea stroma tissue 1110 is removed from the anterior cornea by photoablation. After material removal, the anterior cornea stroma will not regrow, thus resulting in a permanent shape change in the anterior cornea 1120.
Currently, all PRK procedures use an excimer laser to ablate and remove the anterior corneal stroma tissue, PRK provides vision correction with higher corneal mechanical strength than LARK does since there is no flap, and hence, no resulting void created underneath the flap. PRK is typically recommended for patients with thin corneas with a thickness under about 500 μm. PRK is, however, generally considered a more painful procedure with a typical recovery time of two weeks.
At least some prior ophthalmic laser surgery systems can be less than ideal in some instances. For example, prior laser surgery systems for performing a PRK procedure require an excimer laser because of its ability to accurately remove small amounts of corneal tissue. Tissue ablation rates with an excimer laser, however, can vary with the level of corneal hydration. And, to perform LASIK, SmILE, and PRK procedures, both a femtosecond laser surgery system and an excimer laser system are needed, thereby increasing costs.
For all these reasons, improved methods and systems that overcome at least some of the above limitations of prior systems and methods are desired.